THE STEAM TURBINE—PARSONS. 101 
ume. At the opposite end to the blade drums are seen the balance 
pistons, or dummy drums, which serve to balance the end pressure 
of the steam, and are kept steam-tight with the casing by packing 
grooves on the dummy drums which rotate in close proximity to 
corresponding but stationary brass rings keyed into the case. 
In land turbines, for driving dynamos or other fast moving ma- 
chinery, no end-pressure on the shaft is required, nor is it permis- 
sible because of the mechanical difficulties met with in thrust-bearings 
‘arrying heavy end-pressure and rotating at high speed, and there- 
fore balance pistons are provided, which, while being practically 
steam-tight, serve to balance all end-pressure arising from the steam 
acting upon the rotating barrels and vanes. 
In marine turbines, on the other hand, the dummy drunts are so 
proportioned as to leave an unbalanced end-pressure, which counter- 
acts and balances the thrust of the propeller, thus relieving the thrust- 
bearing from. pressure. 
The bearings of the engine, it will be seen, have only to support 
the weight of the rotating part of the engine; this is comparatively 
small, and as continuous lubrication is provided by an oil pump 
which circulates the oil continuously through the journals round and 
round, there is practically no wear, even after years of continuous 
work; and the maintenance of the shaft in a truly central position 
relatively to the casing, which is of great importance, is easily main- 
tained in practice. 
Before proceeding further with the examination of the compound 
steam turbine, let us consider the De Laval steam turbine introduced 
by Doctor De Laval, of Stockholm, in 1888. 
In this turbine the steam at full pressure issues from a diverging 
conical jet, so formed and proportioned that the steam after passing 
through the neck of the jet enters a gradually divergent passage of 
imcreasing cross-section, in which it expands; the result being that 
nearly the whole available energy in the steam is utilized in impart- 
ing to it a very high velocity, reaching, with 100-pound boiler pres- 
sure and a good vacuum, as much as 4,200 feet per second, and the 
discovery of this property of the expanding jet is due chiefly to 
Doctor De Laval. 
This rapidly moving column of expanded steam is directed against 
cupped steel buckets on the periphery of a wheel made of the 
strongest steel, the wheel being shaped so as to permit of the highest 
peripheral velocity consistent with safety, which may be from 800 to 
1,200 feet per second; the steam, by striking the cups and reacting, 
partly by velocity of flow and partly by elastic gaseous rebound 
from the concave surface of the cups, leaves the wheel with a con- 
